1. Field of the Invention
The present invention relates to a liquid crystal display device, more particularly to a liquid crystal display device having dummy electrodes.
2. Description of the Related Art
Liquid crystal display devices using liquid crystal are widely used recently. FIG. 12 is a simplified sectional view of a conventional liquid crystal display device 16a.
The liquid crystal display device 16a has a cell in which a pair of substrate members 7a, 7b are disposed apart from each other through a sealing member 5. The substrate member 7a includes a plurality of signal electrodes 3 on one surface 1a of a substrate 1 and the substrate member 7b includes a plurality of scanning electrodes 4 on one surface 2a of a substrate 2. Orientation films are formed on the respective surfaces 1a, 2a of the substrates 1, 2. The surfaces 1a, 2a including the signal electrodes 3 and the scanning electrodes 4, respectively confront each other, and liquid crystal is charged into the space enclosed by the surfaces 1a, 2a and the sealing member 5, thereby forming a liquid crystal layer 6.
In the liquid crystal display device 16a of such a constitution, an interval d2 between a portion having the signal electrodes 3 and a portion having the scanning electrodes 4, and an interval d1 between portions without electrodes 3, 4 are different, and hence the liquid crystal layer 6 placed between the pair of substrate members 7a, 7b is not uniform in thickness. In the liquid crystal display device 16a, since the color of the display screen of the device 16a varies, when voltage is not applied to the signal electrodes 3 and scanning electrodes 4, depending on the thickness of the liquid crystal layer 6, an uneven color occurs in the display screen if the liquid crystal layer 6 is not uniform in thickness as stated above.
A prior art for preventing this uneven color is disclosed, for example, in Japanese Unexamined Patent Publication JPA 62-229234 (1987). In the prior art, signal electrodes 3 and scanning electrodes 4 are provided on mutually confronting surfaces 1a, 2a of the pair of substrates 1, 2, and dummy electrodes having the same thickness as that of the electrodes 3, 4 are provided in the regions where electrodes 3, 4 on the surfaces 1a, 2a do not exist, thereby preventing the thickness of the liquid crystal layer 6 from being uneven and decreasing uneven color,
Since the dummy electrodes are electrically isolated, the dummy electrodes may be charged by static electricity generated in the liquid crystal display device or the like. When a voltage necessary for display is applied to the signal electrodes 3 or scanning electrodes 4 confronting the dummy electrodes, if the dummy electrodes are charged, malfunction may occur, and undesired display may be made in a region not designed to be used for display.
A prior art for preventing such malfunction is disclosed in Japanese Unexamined Patent Publication JPA 6-51332 (1994) is known. FIG. 13 is a simplified sectional view of a liquid crystal display device 16b according to this prior art. In this prior art, dummy electrodes 12 are provided near a sealing member 13 at the outside of a confronting region of signal electrodes 10 and scanning electrodes 11 provided on surfaces at one side each of a pair of substrates 8, 9, and the dummy electrodes 12 are connected electrically to the confronting signal electrodes 10 or scanning electrodes 11 through a conductive spacer or the like existing in the sealing member 13 so as to be same in potential, thereby preventing malfunction. One substrate member 14 is composed of the substrate 8 and signal electrodes 10, and another substrate member 15 is composed of the substrate 9 and scanning electrodes 11.
Incidentally, Japanese Unexamined Patent Publication JPA 4-97321 (1992), relating to a liquid crystal display device for defining an interval of a pair of substrate members 14, 15 by a support member included in a sealing member 13, discloses an art for making uniform the thickness of liquid crystal layer, in which dummy electrodes are provided in a non-exisiting region of display electrodes such as signal electrodes 10 and scanning electrodes 11 in the end portion of the substrate, and a sealing member 13 is disposed between any two electrodes of signal electrode 10, scanning electrode 11, and dummy electrode, thereby adhering a pair of substrate members 14, 15.
The dummy electrodes 12 used in such a prior art, being disposed near the sealing member 13 and electrically connected with display electrodes such as signal electrodes 10 or scanning electrodes 11 may absorb moisture in the portion contacting with the atmosphere, for example, in the condition of use of high temperature and high humidity. When a voltage is applied to display electrodes such as signal electrodes 10 or scanning electrodes 11, an electric field is generated also between dummy electrodes 12 having the moisture absorbing portion and other electrodes 10, 11, and an electrolytic corrosion may occur in the moisture absorbing portion. When the corrosion progresses, the corrosion may also propagate into other display electrodes 10, 11 connected electrically. Variation of resistance value of the display electrodes 10, 11 is caused by the propagation of corrosion, and as a result a difference is caused between the resistance value of the corroded display electrodes 10, 11 and the resistance value of the corrosion-free display electrodes 10, 11, which may cause an uneven color in the display screen of the liquid crystal display device 16b. Moreover, as the corrosion advances, wire breakage or the like may be caused in the display electrodes 10, 11.
As a method of preventing such electrolytic corrosion, it may be considered to apply a moisture preventive agent on the outer circumference of the cell composed of the pair of substrate members 14, 15 and sealing member 13, that is, in the atmosphere contacting portion of the dummy electrodes 12. The moisture preventive agent is applied by pouring into the gap of the portion contacting with the atmosphere between the pair of substrate members 14, 15. At this time, the corners of the liquid crystal display device 16b are longer in the distance, as compared with the peripheral parts, from the edge of the substrate members 14, 15 to the surface portion contacting with the atmosphere of the sealing member 13, and the moisture preventive agent cannot permeate completely into the surface portion of the sealing member 13, and therefore the moisture preventive agent may not be applied to the surface portion of the sealing member 13 and its vicinity. It is hence known difficult to apply completely the moisture preventive agent on the whole circumference of the sealing member 13, especially in the corners.
It may be also considered to remove the portion of the dummy electrodes 12 contacting with the atmosphere in which electrolytic corrosion is likely to occur. In such method, although uneven display by corrosion may be prevented, it is known that uneven color is caused by difference of film thickness near the sealing member 13.